This invention relates to a solvent extraction process useful for the separation of ethanol from water, a principal benefit of which is a low energy requirement.
Certain specific processes which use solvent extraction to separate different organic liquids by selective solvent action are well-known in the art. For example, ethylene is used as a solvent to separate water-organic mixtures in J. C. Elgin and J. J. Weinstock, "Phase Equilibrium At Elevated Pressures In Ternary Systems Of Ethylene And Water With Organic Liquids", J. Chem. and Engr. Data, Vol. 4, No. 1, January, 1959, pp. 3-12. However, ethylene has not been found to be an effective solvent for use in solvent extraction of such mixtures due to the low distribution coefficient of ethylene in water and organic liquid mixtures.
At present, solvents are used to separate ethanol from water only when very high concentrations (90-95% ethanol, by volume) of ethanol and water are involved, because ethanol forms an azeotrope with water at a concentration of about 95% ethanol, by volume, and distillation cannot increase the concentration of ethanol beyond this value. The use of benzene and ethyl ether in the extraction of absolute alcohol from an alcohol-water azeotrope is disclosed in D. F. Othmer and T. O. Wentworth, "Absolute Alcohol--An Economical Method For Its Manufacture", in Industrial and Engineering Chemistry, December, 1940, pp. 1588-1593.
Mann, U.S. Pat. No. 1,524,192 discloses the extraction of a high molecular weight alcohol, such as secondary butyl alcohol, from an aqueous solution, through the utilization of a hydrocarbon oil having a high initial boiling point, preferably above 325.degree. F., as the solvent. Distillation is used to obtain the final dehydrated alcohol product.
In Carney, U.S. Pat. No. 2,048,178, a process for dehydrating organic compounds, such as secondary butyl alcohol, is disclosed. Olefin and parrafin hydrocarbons, such as pentane, isopentene and butane, which are substantially insoluble in water and soluble in the organic compound, are used as solvents. Carney utilizes steam in order to maintain process temperatures and to separate the solvent and organic compounds. This results in high energy costs. In addition, Carney fails to disclose the separation of ethanol from an aqueous medium through the utilization of his process and solvents. In Van Dijck, et al., U.S. Pat. No. 2,081,721, a process for separating a liquid mixture, containing one or more organic polar compounds, into two components or two groups of components, by solvent extraction, is disclosed. The process includes a washing step, in which the separated extract or solvent-rich phase is washed with a liquid stream of a nearly pure component. This process, like the process in Carney, is intended for use in the separation of water from an alcohol higher than ethanol, e.g., propyl and butyl alcohols, by using a hydrocarbon, such as pentane, as the extracting agent.
The feasibility of using solvent extraction as a substitute for distillation in alcohol separation and concentration was studied in J. W. Roddy, "Distribution Of Ethanol-Water Mixtures To Organic Liquids", Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 1, 1981. All of the solvents studied were liquids at ambient temperature and atmospheric pressure.
The prior art separation processes heretofore developed for the separation of ethanol and water have process temperatures or solvent requirements which require large energy inputs and thus high energy costs. The present invention obviates the need for large energy inputs through the use of allene, methyl allene, propylene and methyl acetylene as solvents in a solvent extraction process.
By using these solvents, which are in the vapor state at ambient temperature and pressure, the energy required for their recovery, by reverse osmosis, is minimized. The recovery of the solvents is achieved through the use of a polymer membrane composed of a hydrophobic polymer which preferentially separates hydrogen-bonding from non-hydrogen-bonding molecules. The pressure drop across the membrane provides the driving force to achieve the separation.